introduce Readable and Duplex concat streams

[zenflow]

I wish to be able to do something like this:

concat(input_string) // or `concat([input_string])`
  .pipe(someTransform())
  .pipe(concat(function(data){
    // ...
  });

This is basically using concat-stream to do the reverse (getting a stream from a single piece of data) of what it already does (getting a single piece of data from a stream).

Going even further... why not have duplex concat-streams?

var input = getReadableStream().pipe(concat());
input.once('data', function(data) {
  // do something with [complete] input data
});
var transformed = input.pipe(getTransformStream()).pipe(concat());
transformed.once('data', function(data) {
  // do something with [complete] transformed data
});
transformed.pipe(getWritableStream());

[zenflow]

Relates to #26